Metal oxide nanostructures have been used in a wide variety of different electronic, optoelectronic and electrochemical devices such as sensors, solar cells, lasers, transistors and supercapacitors. In addition to electronic and optical properties, the surface morphology of the nanostructures is critical for implantation in devices.
Dye-sensitized solar cell (DSSC) has long held promise as the next generation solar cell. One direction in that research has been toward the preparation of metal oxide, for example TiO2, electrodes, as efficient DSSC electrodes that require extremely high surface areas to permit sufficient absorption of dye molecule to achieve high photocurrent generation. However, conventional metal oxide particle layers do not provide sufficient physical surface area per unit area to allow high light harvesting by the monolayer of adsorbed dye on the metal oxide surface. Research has primarily been directed to the preparation of mesoporous structures or assemblies of nanoparticles to address the surface area problem.
Concerning metal oxide nanoparticle assemblies, a desired form is densely packed, high aspect ratio vertically aligned nanowires, where promoting increasingly smaller diameters and longer lengths permits enormous surface areas for the process of charge separation. Furthermore, where the individual nanowires are a single crystal, efficient transport of electron to a contacting surface is enhanced.
Performance of devices as supercapacitor strongly depends on the surface area, which limits the charge-storing capacity. Vertically aligned and densely packed metal oxide nanowires grown on a planar substrate increase the specific surface area of an electrode. However, to further increase the specific surface area of an assembly of nanowires, a complex three dimensional topography is desirable. Conceptually, a simple topography for a high area surface would be an array of nanowires radiating from a patterned surface of deep trenches which display a high aspect ratio. Therefore, it is desirable to prepare topography by formation of a structure including metal oxide nanowires radiating from the surfaces of the sidewalls and bottom surfaces of deep trenches to dramatically increase the quantity of metal oxide nanowires and the resulting specific surface area for an electrode.